1. Field of the Invention
This invention relates to water-cooled nuclear reactors and more particularly to apparatus for improving the distribution of coolant entering the core of water-cooled nuclear reactors.
2. Description of the Prior Art
The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated from and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internals structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. The primary side is also connected to auxiliary circuits, including a circuit for volumetric and chemical monitoring of the pressurized water. The auxiliary circuit, which is arranged branching from the primary circuit, makes it possible to maintain the quantity of water in the primary circuit by replenishing, when required, with measured quantities of water, and to monitor the chemical properties of the coolant water, particularly its content of boric acid, which is important to the operation of the reactor.
The average temperature of the core components during full power reactor operation is approximately 580° F. (304° C.). Periodically, it is necessary to shut down the reactor system for maintenance and to gain access to the interior side of the pressure vessel. During such an outage, the internal components of the pressure vessel can cool to a temperature of approximately 50° F. (10° C.). The internal components of a pressure vessel typically consist of upper and lower internals. The upper internals include a control rod guide tube assembly, support columns, conduits for instrumentation which enter the reactor through the closure head, and a fuel assembly alignment structure, referred to as the upper core plate. The lower internals include a core support structure referred to as the core barrel, a core shroud that sits inside the core barrel and converts the circular interior of the barrel to a stepped pattern that substantially corresponds to the perimeter profile of the fuel assemblies that constitute the core supported between the lower core support plate and the upper core support plate.
Generally, the reactor vessel is cylindrical having a hemispherical lower end. The core barrel is connected to the interior walls of the reactor vessel at or adjacent to the area where the cylindrical and hemispherical portions of the reactor vessel meet. Below the main core support, i.e. the core barrel that is capped at its lower end with the lower core support, the hemispherical vessel defines a lower head or lower plenum. A generally annular downcomer surrounds the reactor core barrel between the core barrel and an inner wall of the reactor vessel. Cooling fluid, typically water, is pumped into this annular downcomer. The coolant fluid circulates downward into the lower plenum. The hemispherical shape of the lower plenum assists in evenly circulating the coolant fluid therein. A plurality of reactor core coolant inlet openings are located on the underside of the lower core support plate. Coolant flows from the lower plenum, into the core coolant inlet openings and upwardly into the core to cool the fuel assemblies.
In order to maintain adequate and uniform cooling throughout the core, it is important that a uniform coolant flow and pressure be maintained across all of the reactor core coolant inlet openings in the lower core support plate. Non-uniform coolant pressure or flow causes uneven coolant flow into the core, which results in uneven cooling of the fuel assemblies of the core. Uneven fuel assembly cooling may force the entire core to be derated to accommodate “hot assembly” locations. Non-uniform coolant flow and pressure may result in vortices or other flow disruptions to form in the coolant fluid circulating in the lower plenum.
It is desirable to provide core monitoring instrumentation within the core of a nuclear reactor. Traditionally, the leads connecting such instrumentation to the exterior of the reactor exit the reactor vessel through a central portion of the lower hemispherical portion of the reactor vessel. A plurality of conduits extending from the underside of the lower core plate to the interior walls of the lower hemispherical portion of the reactor vessel carry the instrumentation lines through the lower plenum.
The presence of the conduits in the lower plenum assists in maintaining even coolant flow within the lower plenum and disrupting the formation of vortices in the circulating coolant fluid. Such vortices disrupt coolant flow and produce low pressure areas at the core coolant inlets which they intersect.
In newer reactors, it has become desirable for any instrumentation conduits to exit the reactor vessel other than through the lower plenum. It has been found that the absence of instrumentation conduits from the lower plenum permits vortices to form in the circulating coolant in the lower plenum.
U.S. Pat. No. 5,267,285 issued Nov. 30, 1993 and assigned to the assignee of this invention, suggested the use of one or more spaced parallel plates, supported in the lower plenum parallel to the lower support plate with holes for the passage of coolant, as a means to suppress vortices.
With the advent of larger passive plants with larger cores it became evident that further means were necessary to improve the distribution of coolant flow in the lower plenum to assure uniform coolant flow and pressure were maintained across all of the reactor core coolant inlet openings in the lower core support plate.
Accordingly, there is a further need to improve the design of the lower reactor vessel plenum to assure this uniform flow and pressure is maintained.